This invention relates to a process for producing synthetic natural gas wherein hydrocarbons and steam are reacted in the presence of a nickel-based catalyst at elevated temperatures and pressures in admixture with product gas from the process which is free of carbon dioxide.
Because industrial and communal fuel gas supply systems have been converted to the use of supply natural gas, the conversion of liquid hydrocarbons into methane has become an urgent problem.
Printed German Application 1,063,590 describes a process which serves to produce a rich gas having a high methane content. In the process, predominantly paraffinic hydrocarbons having an average C number of C.sub.4 to C.sub.10 together with 2-5 parts by weight of water vapor per part by weight of hydrocarbons are preheated to a temperature above 350.degree. C. and are then cracked on a nickel-containing catalyst at a temperature in the range of 450-550.degree. C. to form a high-methane gas, which can subsequently be completely reacted at temperatures below 400.degree. C. to form a gas having an even higher methane content, or at temperatures above 550.degree. C. to form a gas having high H.sub.2 and CO contents.
It is known from Opened German Application 1,768,284 to produce high-purity methane by a catalytic hydrocracking of higher hydrocarbons in a process in which hydrogen is added to the hydrocarbon in an amount which is 90-150% of the amount theoretically required to eliminate all C-C bonds, and in which the exothermic effect is reduced and the reaction temperature is restricted to 400.degree.-600.degree. C. in that 0.5-3.5 kilograms water vapor are added per kilogram of the hydrocarbon feedstock. The resulting gas may consist almost entirely of methane and may contain only little hydrogen and carbon dioxide.
These two processes indicate the practical limits within which processes can be developed that serve to convert preferably liquid hydrocarbons into methane.
When it is taken into account that the hydrocracking of liquid hydrocarbons to form methane essentially consists in a saturation of the methylene groups, CH.sub.2, with hydrogen, 1 mole of hydrogen is consumed per C atom. The amounts of hydrogen required to carry out this reaction on a commercial scale can actually be produced only by the known water-gas reactions. As is apparent from the sum formula EQU C + 2 H.sub.2 O = CO.sub.2 + 2 H.sub.2,
1 mole carbon dioxide must be withdrawn as residual gas per 2 moles of hydrogen in the extreme case. It is apparent that 1 mole of carbon dioxide is obtained per 2 methylene groups to be hydrogenated. Besides, an extensive plant is required for the separate production of hydrogen.
In the production of methane by the cracking of liquid hydrocarbons with water vapor, the oxygen which has been introduced with the water must also be withdrawn from the reaction sequence.
This reaction sequence is a bundle of competing reactions, which in a temperature range of about 450.degree. C. result in a state of equilibrium in which, based on dry gas, methane predominates and carbon dioxide is the second largest component. The remainder consists mainly of hydrogen and small amounts of carbon monoxide.
This reaction system involves the formation of carbon monoxide and hydrogen, the shift conversion of carbon monoxide and water vapor to form carbon dioxide and hydrogen, the hydrogenating cracking of the hydrocarbon feedstock and the hydrogenation of carbon monoxide to methane as well as the Boudouard reaction, in which elementary carbon and carbon dioxide are formed from carbon monoxide.
The Boudouard reaction is the most dangerous competing reaction because the deposition of elementary carbon clogs the catalyst layer and enforces a shutdown.
For this reason, additional process steps have been adopted in all processes of producing methane by a methanation of water gas with the object to minimize the concentration of carbon monoxide in the reaction mixture. In a multi-stage process, the hydrogen is carried through all stages whereas the carbon monoxide is divided into portions which are distributed among the stages. It is also known to recirculate methane-containing product gas through one or more stages and thus to introduce also carbon dioxide into the reaction so that the equilibrium of the Boudouard reaction is pushed to the side of the carbon monoxide.
Rich-gas processes of cracking liquid hydrocarbons with water vapor have been disclosed, e.g., in the abovementioned printed German Application 1,063,590, and it is desired in these processes to produce a cracked gas which contains as much methane as possible, to suppress the formation of carbon black by the Boudouard reaction, if possible, and to minimize the amount of water vapor which is to be added. The water vapor requirement for the reaction is mostly defined as the weight ratio of water vapor to hydrocarbons and will be referred to hereinafter as the water vapor ratio.
It is known that the presence of a surplus of water vapor in the cracking of liquid hydrocarbons with water vapor in the temperature range of 400.degree.-600.degree. C. reduces the tendency to form carbon black but opposes also the formation of methane, and that a lower reaction temperature and/or a higher reaction pressure promote the formation of methane at the expense of the formation of carbon oxides and hydrogen, that an addition of hydrogen to the feedstock mixture consisting of hydrocarbons and water vapor permits a reduction of the water vapor ratio, and by the hydrogenation of carbon oxides or a hydrogenating cracking of the hydrocarbons contributes to the exothermic formation of methane and opposes the formation of carbon black, and that the use of hydrocarbons having a low C number permits a lower water vapor ratio and lower reaction temperatures. To utilize these factors, the known rich-gas process has been modified in various ways, e.g., by a recirculation of product gas (U.S. Pat. No. 3,459,520), by an addition of hydrogen or hydrogen-containing gases (U.S. Pat. No. 3,415,634) or in that the process in carried out in a plurality of stages so that the water vapor is carried through all stages connected in series whereas the hydrocarbons are approximately uniformly distributed among these stages (U.S. Pat. No. 3,420,642).
In the production of high-methane gases by a cracking of liquid hydrocarbons with water vapor on nickel-containing catalysts with performance of individual or several ones of said process steps, the water vapor ratio must not be substantially below 1.6. In that case, the product gas still contains a surplus of water vapor, which is condensed as the gas is cooled. Rich gases thus produced contain on a dry basis at least 67% methane, the balance being carbon dioxide and hydrogen.
To convert such rich gases into a gas which is interchangeable with natural gas and consists mainly of methane and contains no carbon monoxide and only very little hydrogen, the carbon oxides are catalytically hydrogenated to methane so that the remaining hydrogen is consumed, and the still remaining carbon dioxide is scrubbed off.
In this process described in Opened German Specification 1,645,840 that methanation is carried out in two stages. In the stage the primary product consisting of the rich gas inclusive of its water vapor content, is passed over a methanation catalyst, then cooled to condense the water vapor, reheated and passed through the second methanation stage.
Opened German Specifications 1,922,181 and 1,922,182 disclose a process of producing a low-hydrogen, high-methane gas. In that process, a rich gas stage and a methanating stage are connected in series in direct succession, both reactors are indirectly cooled at their outlet end, and one reactant or both reactants, i.e., hydrocarbons and/or water, are introduced in liquid form as a direct-contact cooling fluid into the reaction mixture between the two reactors.